When supply-demand imbalance is caused in a power system, a power system frequency is changed. A generator-motor in the power system increases or decreases an output (active power) according to a detected frequency change width, and suppresses the power system frequency change. In a case of a synchronous generator-motor, if a pump-turbine output is not changed, a generator-motor output is not changed. Therefore, a response speed of the generator-motor output is subject to an influence of a response speed of the pump-turbine output, and the generator-motor output cannot be controlled quickly. Meanwhile, a variable speed pumped storage hydropower system using a doubly-fed generator-motor can control a generator-motor output quickly regardless of a pump-turbine output by using rotational energy. The variable speed pumped storage hydropower system has a great advantage of higher-speed control of a generator-motor output than a synchronous generator-motor.
A rotational speed of the pump-turbine in the variable speed pumped storage hydropower system is controlled at an optimal rotational speed according to a head and an output in order to operate the pump-turbine at a high efficiency point. The variable speed pumped storage hydropower system also has a great advantage of higher system efficiency than a constant speed pumped storage hydropower system and of effective use of water.
A rotational speed change of the generator-motor of the variable speed pumped storage hydropower system is determined by a difference between a generator-motor output and a pump-turbine output. The larger difference between the generator-motor output and the pump-turbine output causes the larger rotational speed change, and the larger difference from the optimal rotational speed.
A conventional control system uses a generator-motor output change amount, which is to be changed quickly, for compensating the demand value of a rotational speed. When the generator-motor output is increased, the generator-motor rotational energy is converted into the output by compensating the rotational speed demand value to be lower. From the viewpoint of efficiency of the pump-turbine, it is desirable to increase the rotational speed when the generator-motor output is increased. However, in the conventional control system, when the generator-motor output is increased, the difference between the generator-motor output and the pump-turbine output becomes larger, and the rotational speed is decreased. There is a drawback of excessive control of the generator-motor output and the rotational speed generator-motor (an overshoot occurs in the generator-motor output and an undershoot occurs in the rotational speed) because of feedback of the generator-motor output and correction of the rotational speed demand value. There is also a drawback of a decrease in pump-turbine efficiency due to a decrease in rotational speed.
Meanwhile, when the output of the generator-motor is decreased, the generator-motor output is converted into rotational energy and the output is decreased quickly by compensating the rotational speed demand value higher. From the viewpoint of efficiency of the pump-turbine, it is desirable to decrease the rotational speed when the generator-motor output is decreased. However, in the conventional control system, when the generator-motor output is decreased, the difference between the generator-motor output and the pump-turbine output becomes larger, and the rotational speed is increased. There is a drawback of excessive control of the generator-motor output and the rotational speed generator-motor (an undershoot occurs in the generator-motor output and an overshoot occurs in the rotational speed) because of feedback of the generator-motor output and correction of the rotational speed demand value. There is also a drawback of a decrease in pump-turbine efficiency due to an increase in rotational speed.